Multilayer printed wiring board and mounting body using the same

ABSTRACT

A multilayer printed wiring board ( 11 ) is composed of a plurality of printed wiring boards ( 21   a  and  21   b ) each having wiring on its both sides, and a relaxing connection layer ( 15 ) for interconnecting the printed wiring boards ( 21   a  and  21   b ). The relaxing connection layer ( 15 ) contains an inorganic filler, a thermosetting resin, and a reliever for relieving internal stress. The multilayer printed wiring board ( 11 ) is prevented from warpage by making the relaxing connection layer ( 15 ) disposed inside it absorb internal stress caused by heating and cooling in a solder reflow process or other processes.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2009/000137, filed on Jan. 16, 2009,which in turn claims the benefit of Japanese Application No.2008-008859, filed on Jan. 18, 2008, the disclosures of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a multilayer printed wiring board usedin various electronic devices such as personal computers, telephones formobile communications, and video cameras, and also relates to a mountingbody using the wiring board.

BACKGROUND ART

Mobile products such as personal computers, digital cameras, and mobilephones are growing in popularity in recent years, and are desired tobecome more compact and more functional.

For example, recent mobile phones are required to have the function oftaking pictures at a high-definition resolution of 2 million pixels ormore.

Patent Document 1 discloses a module with built-in components and acamera module that each include a multilayer circuit board. Suchmodules, which are used on another circuit board like a motherboard, areexpensive, and therefore, it is required to use inexpensive circuitboards.

FIG. 11 is a sectional view of a mounting body in which the cameramodule of Patent Document 1 including a conventional multilayer circuitboard is mounted on a multilayer printed wiring board. In other words,FIG. 11 shows a sectional view of a mounting body for a camera module inwhich multilayer printed wiring board P2 for the camera module ismounted on multilayer printed wiring board P1. Multilayer printed wiringboard P1 can be a laminated body of a glass fiber and an epoxy resin,and can be a motherboard for a mobile phone. Multilayer printed wiringboard P2 can be a multilayer printed wiring board of several millimeterssquare for a camera module. Multilayer printed wiring board P2 may bedesigned for a module with built-in components.

As shown in FIG. 11, large multilayer printed wiring board P1 and smallmultilayer printed wiring board P2 are each composed of a plurality oflayers of wiring 1 stacked through insulating layers 2. Wiring 1 isinterconnected through vias 3. Large multilayer printed wiring board P1and small multilayer printed wiring board P2 are connected via solderballs (not shown).

On the surface of multilayer printed wiring board P2 for the cameramodule, there is fixed image pickup device 5 such as a CCD in such amanner as to be surrounded by holder 4. Image pickup device 5 isconnected to wiring 1 of multilayer printed wiring board P2 throughwires 2 a and other devices. On holder 4, there is fixed lens 6 havingoptical axis 7. As shown by the arrow, light 7 a from outside projects apredetermined image on image pickup device 5 through lens 6. Holder 4supports auto-focusing.

In a mounting body including a multilayer printed wiring board, such asa mounting body for a camera module or for a module with built-incomponents, however, multilayer printed wiring boards P1 and P2 may besubjected to warpage. The warpage is due, for example, to heating andcooling in a solder reflow process during manufacture, or to temperaturechanges in the usage environment. FIG. 12 is a sectional view showingthe warpage problem of multilayer printed wiring boards P1 and P2. Asshown in FIG. 12, multilayer printed wiring boards P1 and P2 tend tohave warpage 8 by their difference in material or the coefficient ofthermal expansion. Warpage 8 means poor flatness, and includesundulation and distortion. As shown in FIG. 12, warpage 8 may result inpeeling 9 a at joints between large multilayer printed wiring board P1and small multilayer printed wiring board P2, for example, at joints ina BGA (Ball Grid Array) package having solder balls. In addition, smallmultilayer printed wiring board P2 and holder 4 may have gap 9 btherebetween, which affects the optical system including lens 6. As aresult, optical axis 7 of lens 6 deviates as shown by optical axis 7 b,making lens 6 out of focus.

Warpage 8 and peeling 9 a may also occur in the case of using a modulewith built-in components instead of a camera module, causing improperconnection of wires or other problems.

As described above, the conventional multilayer printed wiring board andthe mounting body for a camera module using the board have problems suchas optical axis deviation and improper connection due to the warpage andtwist of the multilayer printed wiring board itself.

Patent Document 1; Japanese Patent Unexamined Publication No.2007-165460

SUMMARY OF THE INVENTION

In view of the above-described problems, the present invention isdirected to provide a multilayer printed wiring board which is preventedfrom optical deviation or improper connection of wires that occur in aconventional multilayer printed wiring board when it has warpage, andalso to provide a mounting body using the board.

The multilayer printed wiring board of the present invention includes aplurality of printed wiring boards each having wiring on both sidesthereof; and a relaxing connection layer for interconnecting the printedwiring boards, the relaxing connection layer containing an inorganicfiller, a thermosetting resin, and a reliever for relieving internalstress.

With this structure, the multilayer printed wiring boards are preventedfrom warpage and undulation due to their difference in material or thecoefficient of thermal expansion. Furthermore, a mounting body usingsuch boards has higher precision, and lower cost and weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a mounting body using a multilayer printedwiring board according to a first exemplary embodiment of the presentinvention.

FIG. 2A is a sectional view showing a step of a method for manufacturingthe multilayer printed wiring board according to the first exemplaryembodiment of the present invention.

FIG. 2B is a sectional view showing another step of the method formanufacturing the multilayer printed wiring board according to the firstexemplary embodiment of the present invention.

FIG. 3 shows melt viscosity curves of a relaxing connection layer in themultilayer printed wiring board according to the first exemplaryembodiment of the present invention.

FIG. 4A is a sectional view of the multilayer printed wiring board ofFIG. 1.

FIG. 4B is a partially enlarged sectional view of FIG. 4A.

FIG. 4C is a sectional view of another example of the multilayer printedwiring board of FIG. 1.

FIG. 5A is a sectional view showing a mechanism to reduce warpage of themultilayer printed wiring board according to the first exemplaryembodiment.

FIG. 5B is another sectional view showing the mechanism to reducewarpage of the multilayer printed wiring board according to the firstexemplary embodiment.

FIG. 5C is another sectional view showing the mechanism to reducewarpage of the multilayer printed wiring board according to the firstexemplary embodiment.

FIG. 6A is another sectional view showing the mechanism to reducewarpage of the multilayer printed wiring board according to the firstexemplary embodiment.

FIG. 6B is another sectional view showing the mechanism to reducewarpage of the multilayer printed wiring board according to the firstexemplary embodiment.

FIG. 6C is another sectional view showing the mechanism to reducewarpage of the multilayer printed wiring board according to the firstexemplary embodiment.

FIG. 7A is a sectional view showing a method for manufacturing a POP,which is a mounting body using a multilayer printed wiring boardaccording to a second exemplary embodiment of the present invention.

FIG. 7B is another sectional view showing the method for manufacturingthe POP, which is the mounting body using the multilayer printed wiringboard according to the second exemplary embodiment of the presentinvention.

FIG. 8A is a sectional view showing a step of manufacturing another POPaccording to the second exemplary embodiment of the present invention.

FIG. 8B is a sectional view showing another step of manufacturing thePOP according to the second exemplary embodiment of the presentinvention.

FIG. 9A is a sectional view showing the influence of warpage on a POP.

FIG. 9B is another sectional view showing the influence of warpage onthe POP.

FIG. 10A is a sectional view of an optical module, which is a mountingbody using a multilayer printed wiring board according to a thirdexemplary embodiment of the present invention.

FIG. 10B is another sectional view of the optical module, which is themounting body using the multilayer printed wiring board according to thethird exemplary embodiment of the present invention.

FIG. 11 is a sectional view of a mounting body using conventionalmultilayer printed wiring boards.

FIG. 12 is a sectional view showing the warpage problem of theconventional multilayer printed wiring boards.

REFERENCE MARKS IN THE DRAWINGS

-   11 multilayer printed wiring board-   12, 12 a wiring-   13 via-   14 insulating layer-   15 relaxing connection layer-   16, 16 a, 16 b, 16 c mounting body-   17 holder-   18 image pickup device-   19 lens-   20 optical axis-   21 a, 21 b printed wiring board-   22 wire-   23 adhesive layer-   24 warpage-   24 a, 24 b, 24 c internal stress-   25 semiconductor device-   26, 26 a, 26 b solder ball-   27 crack

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Exemplary Embodiment

A first exemplary embodiment of the present invention will be describedas follows with reference to drawings.

FIG. 1 is a sectional view of a mounting body using a multilayer printedwiring board according to a first exemplary embodiment of the presentinvention.

As shown in FIG. 1, multilayer printed wiring board 11 includes printedwiring boards 21 a and 21 b, and relaxing connection layer 15 connectingthem. Printed wiring boards 21 a and 21 b each have wiring 12 on itsboth sides. Relaxing connection layer 15 includes an inorganic filler, athermosetting resin, and a reliever for relieving internal stress.

Printed wiring board 21 a includes insulating layer 14, which isprovided on its both sides with wiring 12 formed of a copper foilpattern. Insulating layer 14 can be formed, for example, by hardening aglass fiber impregnated with an epoxy resin. Printed wiring board 21 ais provided on its top surface with holder 17 and image pickup device 18surrounded by holder 17 as in the conventional wiring board. Holder 17holds lens 19 having optical axis 20 at its upper part, and supportsauto-focusing (not shown) of lens 17. Wiring 12 on image pickup device18 and wiring 12 on printed wiring board 21 a are connected by wires 22of gold or aluminum by a wire bonder. Wiring 12 on both sides of printedwiring board 21 a is electrically interconnected through vias 13penetrating multilayer printed wiring board 11.

Printed wiring board 21 b, on the other hand, is a laminated bodycomposed of a plurality of printed wiring boards each having wiring 12on its both sides, and a plurality of insulating layers 14 interposedbetween these boards. In short, printed wiring board 21 b itself is amultilayer printed wiring board. Wiring 12 inside and on both sides ofprinted wiring board 21 b is electrically interconnected in the verticaldirection through vias 13 formed in insulating layers 14.

Relaxing connection layer 15 contains a reliever together with theinorganic filler in the thermosetting resin as described in detaillater. The reliever relieves internal stress caused by heating andcooling during the mounting of components on multilayer printed wiringboard 11 (for example, in a solder reflow process). Wiring 12 on oneside of each of printed wiring boards 21 a and 21 b is embedded inrelaxing connection layer 15. Relaxing connection layer 15 has athickness of 30 to 300 μm. Thus, multilayer printed wiring board 11 hasrelaxing connection layer 15 inside so as to reduce surface warpage.

Multilayer printed wiring board 11 is provided on its top surface withlens 19, holder 17, and other optical components so as to form mountingbody 16 for a camera module. The camera module is used in, for example,a mobile phone having a high-definition resolution of 2 million pixelsor more. In short, in the present exemplary embodiment, the componentsmounted on printed wiring board 21 a include holder 17, image pickupdevice 18, and lens 19.

Mounting body 16 can be a POP (Package On Package) or an optical module,besides the camera module. A POP can be composed of a plurality ofmultilayer printed wiring boards connected to each other. An opticalmodule may be composed of a multilayer printed wiring board, and acombined unit of optical cables and circuit components which is mountedon the wiring board. An optical module may alternatively be anoptical-electronic module including a light guide path, an LED (lightemitting diode), or a semiconductor laser formed on the multilayerprinted wiring board.

A method for manufacturing multilayer printed wiring board 11 of thepresent exemplary embodiment shown in FIG. 1 will be described withreference to FIGS. 2A and 2B. FIGS. 2A and 2B are sectional viewsshowing steps of the method for manufacturing the multilayer printedwiring board according to the present exemplary embodiment.

First, as shown in FIG. 2A, the following is prepared: printed wiringboard 21 a having wiring 12 on its both sides; multilayer printed wiringboard 21 b having a plurality of layers of wiring 12 and a plurality ofinsulating layers 14; and relaxing connection layer 15 having vias 13extending between both sides. Predetermined wiring 12 on both sides ofprinted wiring board 21 a is interconnected through vias 13 formed ininsulating layers 14. Wiring 12 inside and on both sides of printedwiring board 21 b are also interconnected through vias 13 formed ininsulating layers 14. Vias 13 in relaxing connection layer 15 are atpositions corresponding to wiring 12 to which they are connected andwhich is formed on both sides of printed wiring boards 21 a and 21 b.

Printed wiring boards 21 a and 21 b can be formed, for example, byhardening a glass fiber with an epoxy resin (which are called glassepoxy boards), or by hardening an aramid fiber with an epoxy resin.Printed wiring board 21 a can be a multilayer printed wiring boardhaving two or more layers, instead of being a single-layer double-sidedprinted wiring board. Vias 13 in printed wiring boards 21 a and 21 b canbe formed by plating (filled vias or plated through holes) instead ofusing an after-mentioned conductive paste.

Relaxing connection layer 15 is an insulating layer containing aninorganic filler, a thermosetting resin, and a reliever. Relaxingconnection layer 15 is first perforated with holes using a laser or adrill, and then the holes are filled with a conductive paste (forexample, a via paste made of copper power and epoxy resin) so as to formvias 13 as shown in FIG. 2A. In the present exemplary embodiment,relaxing connection layer 15 has a thickness of 30 to 300 μm.

Next, relaxing connection layer 15 is sandwiched between printed wiringboards 21 a and 21 b, and is applied with heat and pressure as shown bythe arrows of FIG. 2A. As a result, relaxing connection layer 15 issoftened, becoming more adhesive with printed wiring boards 21 a and 21b. In FIG. 2A, printed wiring boards 21 a and 21 b each have wiring 12 aout of wiring 12 on its side that faces relaxing connection layer 15.Wiring 12 a is aggressively embedded in relaxing connection layer 15,improving the electrical connectivity with vias 13 in relaxingconnection layer 15, and hence, reducing electrical resistance.

In FIG. 2A, vias 13 are projected from the surface of relaxingconnection layer 15 by using, for example, a PET (polyethyleneterephthalate) film, which will be described later. This increases theconnection stability between relaxing connection layer 15 and wiring 12on both sides of printed wiring boards 21 a and 21 b when they arestacked with pressure.

FIG. 2B is a sectional view of multilayer printed wiring board 11 gonethrough the step shown in FIG. 2A. In the step of FIG. 2A, wiring 12 aon one side of printed wiring boards 21 a and 21 b is embedded in vias13 of relaxing connection layer 15 when applied with heat and pressureas shown by the arrows. Thus, wiring 12 a is embedded throughout itsthickness in vias 13 of relaxing connection layer 15. This improves theelectrical connectivity between wiring 12 a and vias 13 of relaxingconnection layer 15.

Holder 17, image pickup device 18, lens 19, and other components aremounted on the top surface of multilayer printed wiring board 11 shownin FIG. 2B so as to obtain mounting body 16 for the camera module shownin FIG. 1, which can be used in a mobile phone.

The following is a description of various components of multilayerprinted wiring board 11. Relaxing connection layer 15 in the presentexemplary embodiment is an insulating layer in which an inorganic fillerand a reliever are dispersed in a thermosetting resin such as an epoxyresin. The reliever can be, for example, a thermoplastic resin such asan elastomer or a rubber.

Relaxing connection layer 15 preferably has a thickness of 30 to 300 μm.When the thickness is less than 30 μm, wiring 12 cannot be embeddedenough to have excellent conductivity with vias 13. When the thicknessexceeds 300 μm, on the other hand, vias 13 cannot be made compact tomaintain their aspect ratio, or become less conductive with wiring 12due to their large thickness.

Relaxing connection layer 15, which is an insulating layer, generallycontains a core material of woven or nonwoven cloth, or film. In thepresent exemplary embodiment, however, relaxing connection layer 15contains a small amount, preferably not more than 5% wt, of corematerial of woven or nonwoven cloth, or film. When the amount exceeds 5%wt, the core material prevents the internal stress from being relaxed,possibly making relaxing connection layer 15 less effective to preventwarpage. In addition, it becomes difficult to embed wiring 12 on oneside of each of printed wiring boards 21 a and 21 b in relaxingconnection layer 15. To ensure the embedding of wiring 12, relaxingconnection layer 15 preferably does not contain a core material of wovenor nonwoven cloth, or film. In other words, relaxing connection layer 15is preferably composed exclusively of an inorganic filler, athermosetting resin, and a reliever.

In the present exemplary embodiment, the inorganic filler contained inrelaxing connection layer 15 is preferably made at least one of silica,alumina, and barium titanate. The inorganic filler preferably has aparticle size of 0.1 to 15 μm. To prevent the conductive paste powder(which means powder contained in a conductive paste) from flowing, theinorganic filler is preferably smaller in particle size than theconductive paste. The conductive paste has a particle size of 1 to 20μm, which is a normal size, and therefore, the practical particle sizeof the inorganic filler is 0.1 to 15 μm. Relaxing connection layer 15preferably has an inorganic filler content of 70 to 90 wt %. When theinorganic filler content is less than 70%, relaxing connection layer 15becomes coarse due to the small amount of the inorganic filler comparedwith the amount of the thermosetting resin. This causes the inorganicfiller to be flown together with the thermosetting resin during thepressing process. When the content exceeds 90%, on the other hand,relaxing connection layer 15 has too low a resin content to allow thewiring to be fully embedded therein or to have sufficient adhesionthereto.

The elastomer used for the reliever in the present exemplary embodimentis either an acrylic elastomer or a thermoplastic elastomer. Morespecifically, the elastomer can be, for example, a polybutadiene- orbutadiene-based random copolymer rubber, or a copolymer containing hardand soft segments. The elastomer content of the epoxy resin compositionis preferably 0.2 to 5.0 wt %. When the elastomer content is less than0.2 wt %, the resin flow increases, whereas when it exceeds 5.0 wt %,relaxing connection layer 15 increases its elasticity, making vias 13less reliable.

In relaxing connection layer 15 in the present exemplary embodiment, thethermosetting resin contains the inorganic filler and either anelastomer or a rubber dispersed therein. This allows the elastomer tosegregate to the surface of the inorganic filler, further reducing thefluidity of the inorganic filler.

The conductive paste to be filled into vias 13 of relaxing connectionlayer 15 in the present exemplary embodiment is preferably made at leastone of copper, silver, gold, palladium, bismuth, tin, and alloysthereof, and has a particle size of 1 to 20 μm. The conductive pastegenerally has a particle size of 1 to 8 μm, but may have a maximumparticle size of 20 μm depending on the variations in particle sizedistribution. Therefore, it is preferable to use the most availableconductive paste, which has a particle size of 1 to 20 μm.

The above-described conductive paste is injected into relaxingconnection layer 15 as follows. First, a PET film is pasted on bothsides of relaxing connection layer 15. Then, through-holes are formed toextend between the two PET films. Next, the through-holes are filledwith the conductive paste, thereby forming vias 13. As a result,relaxing connection layer 15 has the PET films on its both sides andvias 13 therein.

Relaxing connection layer 15 having the PET films thereon is pasted onprinted wiring boards 21 a and 21 b as follows. First, the PET film onone side of relaxing connection layer 15 is peeled off, and relaxingconnection layer 15 is applied to printed wiring board 21 b. Then, theother PET film is peeled off, and printed wiring board 21 a is appliedto relaxing connection layer 15. The reason that the PET films on bothsides of relaxing connection layer 15 are not peeled off at one time isthat non-hardened relaxing connection layer 15 can be easily broken andis therefore difficult to handle. For this reason, one PET film oneither side is peeled off before stacking. Thus, multilayer printedwiring board 11 is completed as shown in FIG. 2B.

As shown in FIGS. 2A and 2B, relaxing connection layer 15 and printedwiring boards 21 a and 21 b are stacked in such a manner that theconductive paste used for vias 13 of relaxing connection layer 15 ispressed with heat onto wiring 12 on one side of each of printed wiringboards 21 a and 21 b. During this stacking process, wiring 12 isembedded in relaxing connection layer 15. As a result, the conductivepaste is further compressed, greatly increasing the conductivity withwiring 12.

Relaxing connection layer 15 preferably has a glass transition point Tg(measured by Dynamic Mechanical Analysis) of either not lower than 185°C. or higher by 10° C. or more than that of printed wiring boards 21 aand 21 b, which are respectively on and under relaxing connection layer15. When the glass transition point Tg of relaxing connection layer 15is either not lower than 185° C. or different by less than 10° C. fromthat of printed wiring boards 21 a and 21 b, these boards may havecomplex or irreversible warpage or undulation in a high temperatureprocess such as reflow.

FIG. 3 shows melt viscosity curves of relaxing connection layer 15according to the present exemplary embodiment. The curve “a” and thecurve “b” show the uppermost value and the lowermost value,respectively, of relaxing connection layer 15 with the above-describedconditions. The curve “c” and the curve “d” show the upper value and thelower value, respectively, of relaxing connection layer 15 with theabove-described conditions. Thus, the lowest melt viscosity of relaxingconnection layer 15 is appropriately 1000 to 100000 Pa·s as shown by thearrow in FIG. 3. When the lowest melt viscosity is less than 1000 Pa·s,the resin flow increases. When the lowest melt viscosity exceeds 100000Pa·s, on the other hand, it becomes impossible to fully bond relaxingconnection layer 15 to the printed wiring boards or to embed the wiringtherein.

Relaxing connection layer 15 may contain a coloring agent, whichimproves mounting properties and light reflectivity of an opticaldevice.

The thermosetting resin contained in relaxing connection layer 15 can beat least one selected from an epoxy resin, a polybutadiene resin, aphenol resin, a polyimide resin, a polyamide resin, and a cyanate resin.

Relaxing connection layer 15 is never formed on the outermost ofmultilayer printed wiring board 11. If done so, relaxing connectionlayer 15 does not exhibit the effect of relaxing the internal stressbetween printed wiring boards 21 a and 21 b. Relaxing connection layer15 can be used as a layer to bond between printed wiring boards 21 a and21 b stacked together so as to obtain a high production yield and toreduce costs.

Relaxing connection layer 15 in the present exemplary embodimentpreferably has a coefficient of thermal expansion of not higher than 65ppm/° C., and preferably lower than printed wiring boards 21 a and 21 b.When the coefficient of thermal expansion of relaxing connection layer15 is higher than either that of printed wiring boards 21 a and 21 b or65 ppm/° C., relaxing connection layer 15 may not sufficiently absorbthe internal stress caused by heating and cooling.

Printed wiring boards 21 a and 21 b may be any resin boards such asthrough-hole wiring boards, or ALIVH (any layer inner via hole) boards.Printed wiring boards 21 a and 21 b may be either double-sided ormultilayer boards. It is also possible to stack printed wiring boards 21and a plurality of relaxing connection layers 15 alternately on eachother.

The insulating material used for printed wiring boards 21 a and 21 b inthe present exemplary embodiment is a composite of a glass woven fabricand an epoxy resin. Alternatively, the insulating material may be acomposite of a thermosetting resin and an organic woven fiber of eitheraramid or wholly aromatic polyester. Further alternatively, theinsulating material may be a composite of a thermosetting resin and anonwoven cloth that is either a glass fiber or an organic fiber selectedfrom p-aramid, polyimide, poly-p-phenylene benzobisoxazole, whollyaromatic polyester, PTFE, poly(ethersulfone), and polyetherimide.Further alternatively, the insulating material may be a composite of asynthetic resin film and a thermosetting resin layer formed on bothsides thereof. The synthetic resin film can be at least one of p-aramid,poly-p-phenylene benzobisoxazole, wholly aromatic polyester,polyetherimide, polyetherketone, polyetheretherketone, polyethyleneterephthalate, polytetrafluoroethylene, polyethersulfone, polyesterterephthalate, polyimide, and polyphenylene sulfide.

Mounting body 16 for the camera module manufactured as described aboveis shown in FIG. 1. In FIG. 1, multilayer printed wiring board 11includes relaxing connection layer 15 as its inner layer so as to reducesurface warpage. As a result, mounting body 16 including multilayerprinted wiring board 11 as shown in FIG. 1 is prevented from warpagewhen the internal stress is caused by heating and cooling in, forexample, a solder reflow process. Therefore, when mounting body 16 isreflow soldered on another printed wiring board such as the motherboardfor a mobile phone, holder 17 and multilayer printed wiring board 11 donot have warpage 8 or gap 9 b therebetween as shown in FIG. 12,regardless of their difference in the coefficient of thermal expansion.In the same manner, no warpage 8 or peeling 9 a shown FIG. 12 occursbetween mounting body 16 and the other printed wiring board on whichmounting body 16 is mounted.

Thus, multilayer printed wiring board 11 including relaxing connectionlayer 15 as its inner layer is prevented from warpage, which is likelyto occur in a solder reflow process.

FIG. 4A is a sectional view of multilayer printed wiring board 11 of thepresent exemplary embodiment shown in FIG. 1. FIG. 4B is an enlargedsectional view of an area “P” of FIG. 4A.

In FIG. 4A, multilayer printed wiring board 11 is an integratedcombination of two-layer printed wiring board 21 a, four-layer printedwiring board 21 b, and relaxing connection layer 15 sandwichedtherebetween. As shown in FIG. 4B, wiring 12 a on one side of each ofprinted wiring boards 21 a and 21 b is embedded in relaxing connectionlayer 15. Wiring 12 a on one side of each of printed wiring boards 21 aand 21 b is thus embedded in relaxing connection layer 15 so as tocompress vias 13 of relaxing connection layer 15 to a degreecorresponding to the thickness of wiring 12 a. As a result, thereliability of via 13 is improved, thereby reducing the electricalresistance.

FIG. 4C shows multilayer printed wiring board 11, which is an integratedcombination of four-layer printed wiring board 21 a, four-layer printedwiring board 21 b, and relaxing connection layer 15 sandwichedtherebetween. This eight-layer multilayer printed wiring board 11 isformed by combining separately prepared non-defective four-layer printedwiring boards 21 a and 21 b with a relaxing connection layer sandwichedtherebetween. This provides a higher production yield than in the caseof forming eight-layer printed wiring board 11 at one time.

In FIGS. 1 and 4A to 4C, relaxing connection layer 15 includes vias 13,but vias 13 are not essential. A similar effect can be obtained as longas a plurality of printed wiring boards are stacked on each other with arelaxing connection layer therebetween.

As described above, in the present exemplary embodiment, multi- orsingle-layer printed wiring boards 21 a and 21 b can be integrated witheach other with relaxing connection layer 15 sandwiched therebetween tobe prevented from warpage during heating and cooling.

A mechanism to reduce warpage of the multilayer printed wiring board 11of the present exemplary embodiment will be described as follows withreference to FIGS. 5A to 5C and 6A to 6C. FIGS. 5A to 5C are sectionalviews showing the mechanism to cause warpage of multilayer printedwiring board 11 including an ordinary adhesive layer.

In FIG. 5A, multilayer printed wiring board 11 is an integratedcombination of printed wiring boards 21 a and 21 b each having wiring 12on its both sides and conventional adhesive layer 23 disposedtherebetween. Adhesive layer 23 can be, for example, a prepreg formed byimpregnating a commercially available glass fiber with an epoxy resin.

FIG. 5B is a sectional view of multilayer printed wiring board 11 inwhich wiring 12 on one side of each of printed wiring boards 21 a and 21b is embedded in conventional adhesive layer 23 shown in FIG. 5A. Asshown by the arrows of FIG. 5B, printed wiring boards 21 a and 21 b aresubjected to internal stresses (or extensions) 24 a and 24 b duringheating of multilayer printed wiring board 11. The lengths of the arrowsindicate the magnitudes of internal stresses 24 a and 24 b (or theamounts of extensions of the boards during heating). In FIG. 5B,internal stress 24 a is greater than internal stress 24 b (internalstress 24 a>internal stress 24 b). This phenomenon occurs, for example,when printed wiring board 21 a has a higher coefficient of thermalexpansion than printed wiring board 21 b. Printed wiring board 21 agreatly extends as shown by internal stress 24 a, which causes adhesivelayer 23 to extend in the same manner.

FIG. 5C shows printed wiring boards 21 a, 21 b subjected to internalstresses (or shrinkage) during cooling of multilayer printed wiringboard 11, opposite to the case of FIG. 5B. As shown by internal stress24 c, a printed wiring board having a higher coefficient of thermalexpansion (for example, printed wiring board 21 a) shrinks greatly. As aresult, as shown in FIG. 5C, multilayer printed wiring board 11 issubjected to convex (or concave) warpage 24 in its bottom (or top)surface. The reason for this is that conventional adhesive layer 23 isthermoset during heating shown in FIG. 5B.

The magnitude, direction, and degree of warpage 24 depend on variousconditions of printed wiring boards 21 a and 21 b. Warpage 24 is causedby small differences between printed wiring boards 21 a and 21 b, suchas the variations in material, thickness, or copper foil pattern size.These differences are due to the difference in the coefficient ofthermal expansion between wiring 12 made of copper foil and insulatinglayers 14 made of a glass epoxy resin or other resins. Furthermore, evenwithin a single printed wiring board, warpage is different from part topart according to the difference in the degree of fineness of wiring 12.Warpage can also occur according to the difference in the number oflayers between printed wiring boards 21 a and 21 b. For example, asix-layer printed wiring board consisting of a two-layer board and afour-layer board is likely to be subjected to the internal stressesshown in FIG. 5B or 5C. Also, the difference in the remaining copperrate affects the warpage. For example, the larger the remaining copperrate, the more the coefficient of thermal expansion of the copper foilis dominant.

FIGS. 5A to 5C show the case in which warpage 24 is caused by heatingand cooling during manufacture, but may be caused by heating and coolingeven after manufacture (after multilayer printed wiring board 11 iscompleted as a product). Warpage 24 is caused by the difference in thecoefficient of thermal expansion between hardened conventional adhesivelayer 23 and printed wiring boards 21 a and 21 b.

The following is a description, with reference to FIGS. 6A to 6C, of themechanism to prevent warpage of multilayer printed wiring board 11including relaxing connection layer 15.

In FIG. 6A, multilayer printed wiring board 11 is an integratedcombination of printed wiring boards 21 a and 21 b each having wiring 12on its both sides and relaxing connection layer 15 disposedtherebetween.

FIG. 6B is a sectional view of multilayer printed wiring board 11 inwhich wiring 12 on one side of each of printed wiring boards 21 a and 21b is embedded in relaxing connection layer 15 shown in FIG. 6A. As shownin FIG. 6B, printed wiring boards 21 a and 21 b are subjected tointernal stresses (or extensions) 24 a and 24 b during heating ofmultilayer printed wiring board 11, for example, in a solder reflowprocess. The lengths of the arrows indicate the magnitudes of internalstresses 24 a and 24 b (or the amount of extensions of the boards duringheating). In FIG. 6B, internal stress 24 a is greater than internalstress 24 b (internal stress 24 a>internal stress 24 b). This phenomenonoccurs, for example, when printed wiring board 21 a has a highercoefficient of thermal expansion than printed wiring board 21 b. In thepresent exemplary embodiment, however, when internal stress 24 a occursin printed wiring board 21 a, it is relaxed by relaxing connection layer15.

FIG. 6C shows printed wiring boards 21 subjected to internal stresses(or shrinkage) during cooling of multilayer printed wiring board 11. Asshown in internal stress 24 c, a printed wiring board having a largercoefficient of thermal expansion (for example, printed wiring board 21a) shrinks greatly. In the present exemplary embodiment, however,internal stress 24 c is relaxed by relaxing connection layer 15.

The above-mentioned heating and cooling are performed, for example, inthe following situations: when the components of multilayer printedwiring board 11 shown in FIGS. 6B and 6C are stacked with heat, or whenvarious electronic components (such as a chip part and a semiconductor)are reflow soldered on completed multilayer printed wiring board 11.Especially in recent years, lead-free solder used for solderingincreases the solder reflow temperature, and hence, the temperaturerange of heating and cooling, thereby making warpage more likely tooccur than before. In the present exemplary embodiment, however, warpageof multilayer printed wiring board 11 is prevented by the function ofrelaxing connection layer 15 to absorb the internal stress occurringduring heating and cooling.

Since relaxing connection layer 15 has a lower coefficient of thermalexpansion than printed wiring boards 21 a and 21 b, the occurrence ofwarpage due to the difference in thermal expansion between printedwiring boards 21 a and 21 b is reduced even when multilayer printedwiring board 11 is heated or cooled after manufacture. Relaxingconnection layer 15 has a higher glass transition point Tg, and hence,higher elasticity than printed wiring boards 21 a and 21 b in thetemperature range for reflow. The temperature range is, for example,from room temperature to 260° C. As a result, relaxing connection layer15 has the effect of reducing warpage of the entire multilayer printedwiring board.

Second Exemplary Embodiment

FIGS. 7A and 7B are sectional views showing a method for manufacturing aPOP, which is a mounting body using a multilayer printed wiring boardaccording to a second exemplary embodiment of the present invention.

In FIG. 7A, multilayer printed wiring board 11 is formed by stackingprinted wiring boards 21 a and 21 b on and under relaxing connectionlayer 15. Printed wiring boards 21 a and 21 b are single layers, andeach have wiring 12 on its both sides which is interconnected throughvias 13. As described in the first exemplary embodiment, in heating andcooling processes for manufacturing multilayer printed wiring board 11,the difference in the coefficient of thermal expansion between printedwiring boards 21 a and 21 b is influential. As described above, thedifference in the coefficient of thermal expansion between printedwiring boards 21 a and 21 b is due, for example, to the variations inthe remaining copper rate, the coefficient of thermal expansion of thewiring boards, and the degree of fineness of wiring 12. In the presentexemplary embodiment, however, as described in the first exemplaryembodiment, relaxing connection layer 15 absorbs the influence of thedifference in the coefficient of thermal expansion between printedwiring boards 21 a and 21 b. As a result, multilayer printed wiringboard 11 is not affected by the warpage caused in the heating andcooling processes. Next, semiconductor device 25 is positioned and fixedas shown by the arrow on multilayer printed wiring board 11.

Then, as shown in FIG. 7B, wiring 12 on semiconductor device 25 isconnected to wiring 12 on printed wiring board 21 a through wires 22 andother devices, thereby completing mounting body 16. In the presentexemplary embodiment, the components mounted on multilayer printedwiring board 11 include semiconductor device 25, wiring 12, and wires22.

Mounting body 16 can be an interposer (a kind of circuit boardinterposed between fine semiconductor chips and a general printed wiringboard) or a package for a POP. Mounting body 16 is connected to anotherprinted wiring board such as a motherboard by reflow soldering viasolder balls 26 formed on the bottom surface of printed wiring board 21b.

To achieve a high density motherboard for a mobile phone or othersimilar devices, it is preferable to form a POP on the motherboard.Using a POP can reduce the number of solder mounting processes, forexample, to one time, thereby increasing the productivity of a mobilephone or other similar devices. Solder mounting, however, tends to causewarpage of printed wiring boards used in a POP (a printed wiring boardon which a semiconductor is mounted, or a printed wiring board as amotherboard) in a solder reflow process.

In the case in which mounting body 16 is mounted on a motherboard viasolder balls 26 as shown in FIG. 7B, a conventional BGA has a pitch of 1mm or so, and solder balls 26 have a large diameter (for example, 800μm). In recent years, however, BGAs have a larger number of IOs (inputsand outputs) and packages are more compact than before. This makes itnecessary to form a large number of solder balls 26 in a limited area,thereby decreasing their diameter (for example, 500 μm or smaller). As aresult, even a slight warpage of printed wiring board 11 mounted on themotherboard has great influence on mounting properties. The warpage,however, can be reduced by using multilayer printed wiring board 11including relaxing connection layer 15 disposed between printed wiringboards 21 a and 21 b as shown in FIG. 7B.

FIGS. 8A and 8B are sectional views showing steps of manufacturinganother POP. FIG. 8A shows how mounting bodies 16 a and 16 b are stackedon each other via solder balls 26. In FIG. 8A, mounting body 16 b isobtained by the method shown in FIGS. 7A and 7B. Mounting body 16 a, onthe other hand, is composed of single insulating layer 14 having wiring12 on its both sides, and semiconductor device 25 formed on the topsurface thereof. Wiring 12 on the top surface of mounting body 16 b isconnected to solder balls 26 of mounting body 16 a, and mounting body 16b is stacked on mounting body 16 a as shown by the arrow, therebyobtaining mounting body 16 c shown in FIG. 8B as a POP. Alternatively,not only mounting body 16 b, but also mounting body 16 a can be formedusing a multilayer printed wiring board including relaxing connectionlayer 15 to improve their mounting properties and to use smaller solderballs 26.

Mounting body 16 c of FIG. 8B is mounted as shown by the arrow onanother circuit board (not shown) such as a motherboard for a mobilephone.

The influence of warpage on the POP will be described as follows withreference to FIGS. 9A and 9B. FIG. 9A is a POP, which is obtained by theconventional method and has the same structure as the mounting bodyshown in FIG. 8B. In FIG. 9A, the POP as a mounting body is composed ofmultilayer printed wiring board 21 b, and single-layer printed wiringboard 21 a stacked on the top surface thereof via solder balls 26 a. Insingle-layer printed wiring board 21 a, wiring 12 on the surface ofinsulating layer 14 is connected to semiconductor device 25 a throughwires 22. Wiring 12 on both sides of insulating layer 14 isinterconnected through vias 13 penetrating single-layer printed wiringboard 21 a. In multilayer printed wiring board 21 b, wiring 12 on thesurface of a laminated body of three insulating layers 14 is connectedto semiconductor device 25 b through wires 22. Wiring 12 inside and onboth sides of multilayer printed wiring board 21 b is interconnectedthrough vias 13 penetrating multilayer printed wiring board 21 b. Thus,multilayer printed wiring board 21 b obtained by the conventional methodis composed exclusively of insulating layers 14 stacked on each other.Multilayer printed wiring board 21 b and single-layer printed wiringboard 21 a compose the mounting body, which is mounted on anotherprinted wiring board such as a motherboard via solder balls 26 b.

Multilayer printed wiring board 21 b is heated and cooled whensingle-layer printed wiring board 21 a is mounted on multilayer printedwiring board 21 b via solder balls 26 a or when the mounting bodycomposed of single-layer printed wiring board 21 a and multilayerprinted wiring board 21 b is mounted on another printed wiring board viasolder balls 26 b. At this moment, multilayer printed wiring board 21 bis likely to be subjected to warpage 24 as shown in FIG. 9B according tothe remaining copper rate, the presence or absence of wiring patterns,the thickness of the board, and other conditions. As a result, solderballs 26 a may develop cracks 27.

Especially in the case of multilayer printed wiring board 21 b formed byhardening a glass fiber with an epoxy resin, it becomes difficult tomount multilayer printed wiring board 21 b on another printed wiringboard when warpage 24 per 10 mm square exceeds 150 μm. Furthermore,there are other possible problems such as the initial failure orunstable reliability of the mounting body, and the detachment or stretchof solder ball 26 a.

The use, however, of multilayer printed wiring board 11 includingrelaxing connection layer 15 shown in FIGS. 7A and 7B can reduce theamount of warpage per 10 mm square to 150 μm or less. This is becauserelaxing connection layer 15 relaxes the warpage of printed wiring board11. The amount of warpage can be reduced to 30% or less of the diameterof solder balls 26 by controlling the thickness, composition, or otherconditions of relaxing connection layer 15. When the amount of warpageof multilayer printed wiring board 11 is reduced to 30% or less of thediameter of solder balls 26, solder balls 26 can be prevented from wearand cracks 27.

Third Exemplary Embodiment

FIGS. 10A and 10B are sectional views of an optical module, which is amounting body using a multilayer printed wiring board according to athird exemplary embodiment of the present invention. In FIG. 10A,mounting body 16 as an optical module includes optical components, whichare accurately positioned and mounted on the top surface of multilayerprinted wiring board 11 according to the present invention shown inFIGS. 2A and 2B. The optical components include lens 19 fixed to holder17. Lens 19 has optical axis 20. Other unillustrated optical componentsinclude a light-emitting device such as a semiconductor laser and alight-receiving semiconductor device. In the present exemplaryembodiment, the components mounted on multilayer printed wiring board 11include holder 17, lens 19, the light-emitting device, and thelight-receiving semiconductor device. In multilayer printed wiring board11, printed wiring boards 21 a and 21 b are stacked with relaxingconnection layer 15 disposed therebetween and are connected to eachother as shown in FIGS. 2A and 2B. Relaxing connection layer 15 preventswarpage of multilayer printed wiring board 11 in heating and coolingprocesses. As a result, optical axis 20 is not shifted when, forexample, the semiconductor laser is soldered after lens 19 is mounted.This can reduce the adjustment man-hours of the optical module and theproduct cost. Wring 12 inside and on both sides of multilayer printedwiring board 21 b is interconnected through vias 13.

FIG. 10B, on the other hand, shows a mounting body using multilayerprinted wiring board 21 c, which is composed exclusively of insulatinglayers 14 and does not include a relaxing connection layer. As shown inFIG. 10B, in printed wiring board 21 c not including relaxing connectionlayer 15, warpage occurs in heating and cooling processes such assoldering, thereby causing optical axis 20 to be shifted. This increasesvariations in the quality of the optical modules, thereby increasing theadjustment man-hours.

Examples of the optical module include, besides the optical module ofthe present exemplary embodiment, a combined unit of optical cables andcircuit components, and an optical-electronic module including a lightguide path, an LED, or a semiconductor laser formed on the surface of amultilayer printed wiring board. The present invention, however, is notlimited to them.

Besides the above-described POP or optical module in which a pluralityof multilayer printed wiring boards are connected to each other, thepresent invention can also be applied to a camera module for a mobilephone having a high-definition resolution of 2 million pixels or more.In other words, the present invention can be applied to any componentsthat can be mounted on the multilayer printed wiring board of thepresent invention.

As described hereinbefore, according to the present invention, wiring onone side of a printed wiring board is embedded in the relaxingconnection layer containing a reliever which relieves the internalstress caused by heating and cooling. As a result, the printed wiringboard is prevented from warpage or undulation due to its thermalexpansion. Thus, the multilayer printed wiring board is prevented fromwarpage and undulation.

INDUSTRIAL APPLICABILITY

The present invention is useful as a mounting body for a devicerequiring high accuracy, such as a POP, a camera module, or an opticalmodule because of its low warpage.

1. A multilayer printed wiring board comprising: a plurality of printedwiring boards each having wiring on both sides thereof; and a relaxingconnection layer for interconnecting the printed wiring boards, therelaxing connection layer containing an inorganic filler, athermosetting resin, and a reliever for relieving internal stress; and avia formed in the relaxing connection layer, the via being made of aconductive paste.
 2. The multilayer printed wiring board of claim 1,wherein the relaxing connection layer has a thickness of 30 to 300 μm.3. The multilayer printed wiring board of claim 1, wherein the relieveris a thermoplastic resin.
 4. The multilayer printed wiring board ofclaim 1, wherein the reliever is an elastomer or a rubber.
 5. Themultilayer printed wiring board of claim 1, wherein the reliever is apolybutadiene- or butadiene-based random copolymer rubber, or acopolymer containing hard and soft segments.
 6. The multilayer printedwiring board of claim 1, wherein the inorganic filler is made of atleast one of silica, alumina, and barium titanate.
 7. The multilayerprinted wiring board of claim 1, wherein the relaxing connection layerhas an inorganic filler content of 70 to 90 wt %.
 8. The multilayerprinted wiring board of claim 4, wherein the elastomer is an acrylicelastomer or a thermoplastic elastomer; and the relaxing connectionlayer has an elastomer content of 0.2 to 5.0 wt %.
 9. The multilayerprinted wiring board of claim 1, wherein the inorganic filler has aparticle size of 0.1 to 15 μm.
 10. The multilayer printed wiring boardof claim 1, wherein the relaxing connection layer is composedexclusively of the inorganic filler, the thermosetting resin, and thereliever.
 11. The multilayer printed wiring board of claim 1, whereinthe relaxing connection layer contains a coloring agent.
 12. A mountingbody comprising: a multilayer printed wiring board including: aplurality of printed wiring boards each having wiring on both sidesthereof; and a relaxing connection layer for interconnecting the printedwiring boards, the relaxing connection layer containing an inorganicfiller, a thermosetting resin, and a reliever for relieving internalstress; and a component mounted on a surface of the multilayer printedwiring board.